1. Field of the Invention
The invention relates generally to terminal devices capable of outputting image data to external display devices in a supported image format by these external display devices.
2. Description of the Related Art
There is known a system in which an external display device having a larger display screen and a portable terminal device (or a mobile device, a data terminal) having a smaller display screen are interconnected. See, for example, Japanese Patent Application Publication No. 2008-211379.
In the above-stated system, the external display device displays original image data entirely (i.e., as the whole image), while the portable terminal device displays the same original image data in part (i.e., as a selected one of sub-images of the whole image).
More specifically, the portable terminal device divides the whole original image represented by original image data, into a plurality of sub-images, and displays a selected one of the sub-images on a display screen of the portable terminal device.
The thus-configured system allows a user to take a desired action on a particular sub-image displayed on the screen of the portable terminal device, while viewing the whole original image displayed on the screen of the external display device.
FIG. 1 illustrates the configuration of a conventional system in block diagram.
As illustrated in FIG. 1, a portable terminal device 1 and an external display device 2 are interconnected by wire (e.g., via a USB cable) or wirelessly (e.g., via a wireless LAN (Local Area Network), Bluetooth, infrared signals, etc.). The portable terminal device 1 is in the form of, for example, a cellular phone, a mobile phone, a pager or a PDA (Personal Data Assistant), each of which has a display screen having a relatively small area for displaying. The external display device 2 is in the form of, for example, an electronic paper display device, which has a relatively large area for displaying.
The “electronic paper display device” has a display screen of a reflective display type which allows a user to view an image because of reflection of external light. This electronic paper display device is of a non-volatile type which requires electric power only when a displayed image is rewritten or updated.
Many of various types of electronic paper display devices receive text/picture information from other devices such as information terminal devices, and each of such electronic paper display devices acts as if it were a sheet of “paper” on which a text/picture represented by the received data has been printed. Such electronic paper display devices are typically used for displaying still pictures, because of a relatively slow speed of drawing.
Further, the portable terminal device 1 is connected with the Internet via an access network (e.g., a cellular phone network). With the Internet, a server 3 is also connected which has stored original image data. The portable terminal device 1 receives the original image data from the server 3, and then delivers the original image data to the external display device 2. This allows the server 3 to activate the external display device 2 via the portable terminal device 1, to thereby allow the external display device 2 to display the original image data which has been stored in the server 3.
Standard image-formats used for image data transmitted/received over the Internet include, for example, a jpeg (Joint Photographic Experts Group) format which is used for compressing bitmap data. In an example, a terminal device, after receiving compressed image data in a jpeg format, decompresses the compressed image data into bitmap data, and displays an image using the bitmap data.
However, many of existing electronic paper display devices cannot display an image using intact bitmap data which is produced based on three primary colors which are red (R), green (G) and blue (B) (i.e., RGB).
Such electronic paper display devices can display only image data produced in an RGBW modified bitmap format which is defined by, for example, white (W) plus RGB. This requires a server to store the same image content both in a jpeg format and in an RGBW modified bitmap format, for the purpose of allowing, such electronic paper display devices to display the image content supplied from the server.
An alternative approach can be assumed to achieve the same goal in which a portable terminal device such a personal computer decompresses original image data which has been received from a server in a jpeg format, and converts the format of the entire image data, from a jpeg format into an RGBW modified bitmap format, at a time.
A terminal device such as a cellular phone which is low in storage capacity and processing ability (e.g., speed) could not format-convert the entire original image data at a time. In general, a cellular phone is designed to have memory capacity and processing ability enough to support operation of a small-sized display screen mounted in the cellular phone. As a result, it is insufficient for such a cellular phone to process, at a time, a large amount of image data which is to be outputted to an electronic paper display device having a larger display screen than that of the cellular phone.
In an industry of an electronic paper display device, the technological development has promoted an electronic paper display device to be designed to be smaller in thickness and weight than before, which allows the electronic paper display device to be intended as a hand-held display. A situation can be easily assumed in which a user carries with her or him both a cellular phone and such an electronic paper display device together. This could increase a user's need for allowing a terminal device such as a cellular phone to output to an external display device such as an electronic paper display device, image data received from an external device such as a server.
In view of the foregoing, it would be desirable to allow a terminal device, even if it is low in storage capacity and processing ability, to output image data to an external display device such as an electronic paper display device.